Television system



Zjm

4 Sheets-Sheet 1 l I/vvE/vv'on v H Ida /.Bedford A. V. BEDFORD TELEVISION SYSTEM Filed Oct. 26, 1934 Juy 18, 1939.

HIM

juh l, 1939 A. v. BEDFORD TELEVISIN SYSTEM Filed Oct. 26, 1954 4 Sheets-Sheet 2 INVENTOP? Aldauedford H 7 TORNEY 4 Sheets-Sheet 4 n VL L I/vvE/v'ron lola V Bedford muli TIME: 1 n BY my. 116.A

Filed Oct. 26,

airain 2 i' sroN srsraiu Alda V.

.- 0rd, Collingswood, N. J., assignor, by

moana assignments, to Radio Corporation or America, New York, N. Y., a corporation of Delaware Application October 26,

18 Cla..

My invention relates to television systems and particularly to television systems of the type in which a cathode-ray transmitter tube is em- In television systems of the above-mentioned type it has been found that a picture appearing at the receiver usually is too dark in certain areas and too light in other areas. For example, one corner of the picture may be of a very dark shade while the corner diagonally opposite may be of a very light shade. This is commonly referred to as black spot. An investigation has shown that this is mainly the result of a characteristic of the cathode-ray transmitter tube. In addition, hown ever, the use of alternating current lighting for illuminating the object to be transmitted causes a hum component to be superimposed on the picture signal which causes uneven shading of the picture.

In addition to causing unequal shading of the picture, prior to my invention the above-mentioned characteristic of the transmitter tube made it diiiicult to eliminate transients or otherv undesired signals from the output thereof while obtaining maximum signal output.

It is, accordingly, an object of my invention to provide a method of and means for correcting, or otherwise controlling, the shading of a television picture.

It is a further object ci my invention to provide an improved method oi and an. improved means for preventing the transmission of. undesired signals from the picture transmitter.

In avpreferred embodiment of my invention I provide means for mixing with the picture signal saw-tooth and sine waves at the horizontal deecting frequency and harmonics thereof and saw-tooth and sine waves at the vertical deecting frequency and harmonics thereof. I have found that by mixing such correcting waves or signals in the proper phase and amplitude with the picture signal, a signal may be sent out from the transmitter which will produce a properly shaded picture at the receiver.

Also, in a preferred embodiment, I provide a circuit including a clipping tube for removing undesired transients or the like from the output of the cathode-ray transmitter tube and, to facilitate their removal, I mix the above-mentioned correcting signals with the transmitter tube output at a point preceding the clipping tube.

Other objects, features and advantages of my invention will appear from the following description taken in connection with the accompanying drawings, in which;

193%, Serial No. 750,@55

(Ci. l-)

Figure 1 is a circuit and block diagram of a television transmitter embodying my invention,

Fig. 2 is a circuit diagram of the shading amplier shown in Fig. l,

Fig. 3 is a circuit diagram of one of the ampliiers shown in Fig. l.,

Fig. 4 is a circuit diagram of the vertical de= fleeting circuit shown in Fig. l,

Fig. 5 is a schematic diagram of a television receiver,

Figs. 6 and 7 are views representing the possible shading of a picture appearing on the fluores cent screen of the cathode-ray tube receiver, and

Figs. 8 to 17 inclusive are curves Which are referred to in explaining the invention.

Referring to Fig. l, the transmitter includes a cathode-ray transmitter tube l of the type described in an article by V. K. Zworykin published in the January, 1934, issue of the Proceedings ot the Institute of Radio Engineers. comprises an evacuated envelope 3 in which there is located an electron gun consisting of an lndirectly heated cathode 5, a control electrode i and a first anode 9. A second anode it consisting of a metallic coating on the inner surface of the enevelope 3 is provided for accelerating the electrons in the electron beam and for aiding in the focusing of the beam.

.lmosaic i3 of light-senstive elements is so positioned inside the envelope 3 that an optical image, formed by a suitable lens system l2, may be projected thereon whereby capacity elements which are associated with elements of the mosaic are charged in accordance with the intensity of light striking them. This forms on the mosaic what may be referred to as an electrical image. Also, the mosaic is so positioned that it may be scanned by the electron beam.

If an indoor scene is to be transmitted it will generally be desired to illuminate the scene or object, indicated at it, by means of incandescent lamps, such as the lamp i6, which are supplied with current from the 60 cycle power line 25. While the light from such a source varies periodically in intensity whereby the light striking the mosaic I3 causes a hum component in the picture signal, this eiect may be compensated for in my system.

As stated in the above-mentioned publication, the mosaic may be constructed in various ways. In a. preferred construction a mica sheet l5 has a metallic coating or signal plate Il formed on the back side and the mosaic I3 formed on the front side which consists of a very large number of minute silver globules each of which is photo- The tube l sensitized by means of caesium. The light-sensitive silver globules are insulated from each other and each globule has a certain amount of capacity to the metallic coating I1.

In order to scan the mosaic with the electron beam deilecting coils Il may be provided for deiiecting the electron beam vertically and deilecting plates 2I provided for defiecting the beam horizontally. l

A high voltage power supply unit 2l is provided for supplying the various electrodes oi the cathode-ray tube I with the proper voltage. 'I'he input circuit of the power supply unit 2l is connected to a suitable power source, such as a 60- cycle power line 25, while the output circuit is connected to a voltage divider 21. 'I'he cathode 5 is connected to a suitable point on the voltage divider 21 near its negative end, while the control grid 1 is connected to a point on the voltage divider 21 which is negative with respect to the cathode 5. The first and second anodes 9 and II, respectively, are connected to points on the voltage divider which are positive with respect to the cathode l.

A voltage impulse generator 29 is provided which may be the same as that described and claimed in my copending application Serial No. 728,147, filed May 29, 1934, and assigned to the Radio Corporation of America. In this form of impulse generator the impulses are generated by means of a rotatable disc having suitable openings therein through which light may pass to strike a photoelectric cell, the disc being driven by a synchronous motor. As indicated on the drawings, the synchronous motor (not shown) is supplied through conductors Il with current from the 60cycle line 25.

The generator 29 supplies vvertical detlecting impulses through a conductor 33 to a vertical deilecting circuit 35 which supplies a saw-tooth current to the deflecting coils I9. Horizontal defiecting impulses are supplied through a conductor 31 to a horizontal deflecting circuit l! which supplies saw-tooth voltage waves to the deilectin'g plates 2|. In one particular transmitter the vertical deflecting impulses occur 60 times per second, while the horizontal deilecting impulses occur '1290 times per second to lprovide 121% lines per picture, whereby the so-called interlaced scanning is produced. Assuming a picture has been projected onto the mosaic I 3, as the mosaic is scanned from bottom to top (since lthe image is inverted), picture signals will appear across a resistor 4I which is connected be- `tween the signal plate I1 and ground.

With regard to the frequency of the vertical deflecting impulses, it is desirable that they occur at the same frequency as that of the power line 25 so that the uneven shading of the picture, due to the use` of the alternating current light source I8, will remain stationary on the picture.

Consequently, this type of uneven shading may be corrected for in the same manner as the type due to the cathode-ray tube characteristics.

'I'he signals appearing .across the resistor 4I are passed through an a'mpliner 43 and a second amplifier to a radio transmitter 41. The am-y plifler will be described in detail later in connection with Fig. 3.

The vimpulse generator 29 also supplies blanking impulses which are transmitted through a conductor 49 and a buil'er resistor 5I to a suitable point in the amplifier l5. It also supplies synchronizing impulses 5l through a conductor and a butler resistor to the output circuit of the Afrom the other bus bar.

amplifier Il, whereby the synchronizing impulses are transmitted to the receiver together with the picture signals. The manner in which the blanking impulses and synchronizing impulses are utilized will be explained hereinafter,

The term "buffer resistor" is used in the preceding paragraph and will be used in the following paragraphs with reference to a resistor which is employed for the purpose of preventing one circuit from short-circuiting another circuit or otherwise rendering it ineective. In the shading circuit, which is described in the following paragraphs, the buer resistors are employed largely for the purpose of causing voltages of different wave shapes and frequencies to add to give a composite wave of the desired shape.

If only the portion of the transmitter described above were employed for transmitting a picture, the picture appearing on the fluorescent screen at the receiver would be shaded in some such manner as indicated in Figs. 6 and 7. Such uneven shading is particularly noticeable if the intensity of the electron beam is'made sufllcient to obtain maximum picture signal output. With the cathode-ray transmitter tube I illustrated in.

Fig. 1 it has been found that usually the appearance of the picture at the receiver'is similar to the view in Fig. 6. That is, one corner of the picture will be darker than the rest of the picture.

In accordance with my invention, I provide a shading circuit by means of which the picture at the receiver may be given a substantially uniform shading. The main portion of the shading circuit is shown enclosed by the dotted rectangle. 'This portion of the shading circuit supplies to a shading amplifier 51 saw-tooth waves occurring at' the horizontal and vertical deiiectingfrequencies, sine waves of the horizontal and vertical deflecting frequencies, and the second harmonics of these sine waves. The output of the shading amplifier 51 is impressed through a buffer resistor I! upon the input circuitv of the ampliiler I5 where the saw-tooth waves and the sine waves are mined with the picture signal.

Referring more speciilcally to the shading circuit, the saw-tooth waves of the horizontal de- 'iiecting frequency are supplied from the horizontal deilecting circuit 39 through a conductor IIv and through a coupling condenser 63 and buil'er resistor 65 to a potentiometer resistor l1 which is grounded at one end. 'Ihe saw-tooth voltage wave is taken oil' the potentiometer 81 through a slideable contact 89 and transmitted through a resistor 1I to a switch arm 1I which may be connected either to a bus bar 15 or to a bus bar 11.

'I'he bus bars 15 and 11 are so connected through conductors 16 and 18, respectively, to different points in the shading amplifier 51 that a signal supplied to the amplifier from one bus bar will appear in the amplifier output 180l out of phase to a similar signal supplied to the amplifier The manner in which the phase of a shading or correcting signal may be reversed will be understood by referring to` Fig. 2.

As shown in Fig. 2, the shading amplifier 51 comprises three vacuum tubes 1s, II and 83 which maybe' of the suppressor grid type. These ampliiler tubes are connected in cascade in a conventional manner by means of resistance coupling,

each tube being provided with the` proper bias from a self-biasing resistor. It will be seen that the conductor 16 leading from the bus bar 15 The plate circuit of the tube 209 also includes a peaking resistor 2|3 and a condenser 2li which are connectedin series across the plate voltage supply (not shown) andthe plate 'resistor 2|| whereby the condenser 2 I 5 is charged through the two resistors 2|| and 2|3 by the plate supply. Since the condenser 2|5 is being charged through a circuit including a large amount of resistance it is charged in a substantially linear manner until a vertical defiecting impulse is impressed upon the input circuit of the amplier tube 208. This impulse causes the control grid of the tube 204 to become more positive whereby the condenser 2|5 discharges through the said tube. l At the end of this impulse the condenser again receives a substantially linear charge until 'the next vertical deflecting' impulse occurs. During the time of charge, the plate current of the tube 209 is substantially zero because of the negative charge retained on its grid due to electrons collected by said grid during the discharge period. In this manner a saw-tooth vwave voltage Wave is produced across the condenser 2|5. This saw-tooth voltage is impressed through a coupling condenser 2 1 upon the input circuit of an amplifier tube 2 I9. The amplified saw-tooth wave is then supplied through the conductor |35 to the shading circuit as previously described.

The voltage appearing across the condenser 2 I 5 and the peaking resistor 2 I3 is a sum ofthe sawtooth voltage appearing across the condenser 2li and a square top voltage appearing across the peaking resistor 2|3. This voltage is impressed through a coupling condenser 22| and a resistor 223 upon the input circuit of an amplifier tube 225 where it is amplified sufilciently to be impressed across the deiiecting coils H8.

The peaking resistor 2|3 is provided because an impulse component must be added to the sawtooth wave to produce a flow of saw-tooth current in the deflecting coils, this being necessary because of the inductive reactance oi' the deiiecting coils. Since it is generally desirable to round of! the top of the saw-tooth wave for varying the i velocity of the vertical deflection, the input circuit of the amplifier tube 225 includes a resistor 221 and a condenser 229 connected in series whereby the impedance of the input circuit is greater for low frequencies than for high frequencies. The condenser 229 is shunted by a variable resistor 23| in order to control the effectiveness of the condenser 229 in increasing the low frequency response of the amplifier 225. The feature of controlling the velocity of the vertical deection of the cathode ray is described and claimed in my copending application Serial No. 755,304, illed November 30, 193.4, assigned 'to the RCA Victor Company, Inc. l

In Fig. there is illustrated a television receiver for receiving pictures sent out from the transmitter illustrated in Fig. 1. -The television receiver comprises a radio receiver 233 and a separating circuit 235 which separates the picture signals (including the shading signals) from the vertical and horizontal synchronizing impulses and which also separates the vertical synchronizing impulses from the horizontal synchronizing The picture and shading signals areimpulses. passed through a picture amplier 231 and impressed upon the control grid 220 oi' a cathoderay receiver tube 24|.

The cathode-ray tube 24| which `is illustrated is of a well known type comprising an evacuated envelope 242 having an electron gun therein consisting of an indirectly heated cathode 245, the

amavis control grid Ill. and a iirst anode 241. A second anode 249 consisting of a metallic coating on the inner surface oi' the envelope 24S is provided for accelerating the electrons in the electron beam and for aiding in the focusing of the beam. A fluorescent screen 25| is provided at theend of the tube where it may be scanned by the electron beam for producing a picture. In order to deflect the electron beam both horizontally and vertically for scanning purposes, deilecting coils 252 and255. respectively, may be provided.

The horizontal synchronizing impulses are impressed upon a horizontal deecting circuit 251 which supplies to the horizontal deecting coils 252 a saw-tooth current under the control of the synchronizing impulses. 'I'he vertical synchronizing impulses are supplied to a vertical deflecting lcircuit 258 which supplies saw-tooth current to the vvertical deilecting coils 255, this current being under the control of the vertical synchronizing impulses whereby the electron beam in the cathode-ray tube is caused to scan the uorescent screen in synchronism with the scanning of the mosaic I2 at the transmitter.

It will be understood that Figs. 6 and "I are views of the end of the tube 24| when it is being scanned with the transmitter mosaic uniformly illuminated and with no shading or correcting signals being transmitted. It will be noted that the views shown in Figs. 6 and 'I are drawn to a different scale than the cathode-ray tube shown in Fig. 5.

At the transmitter, a monitor receiver is provided as indicated at 258 in Fig. 1. The end of the receiver tube is indicated at 260, the shading on the fluorescent screen being the same as shown in Fig. 6. Except that the radio receiver is omitted. the monitor receiver is the 'same as the receiver illustrated in Fig. 5.

The-.operation of my improved television system will now be described particularly with refy erence to Fig. 3 showing the circuit of the amplifier and with reference to Figs. 8 to 17 showing various voltage curves. In this description of the operation of the shading circuit it will be assumed that, without any correction, a picture appearing on the fluorescent screen 25| is shaded as shown in Fig. 6. This assumption will be made for the sake of simplifying the explanation. although, as 'a matter of fact, it has been found that the shading usually is a combination o! the type of shading shown in Fig. 6 and of that shown in Fig. 7. v

Referring to Fig. 3, the amplifier llcomprises four amplifier tubes 26|, 263, 255 and 261 which are coupled in cascade by means of redstance coupling. All of the tubes are properly biased to function as amplifier tubes either by means of biasing batteries, as illustrated, or in any other suitable manner. It may be desirable to bias the last amplifier tube A261 somewhat more negatively than the preceding tubes in order that it will function more effectively as a clipping" tube as will be understood from the following description.

in Fig. 6, has the characteristic illustrated by the curve 269A in Fig. 8. In order to simplify the curve, only a few lines per picture have been indicated. In this curve the picture signals for each horizontal scanning line are represented by the portions z, while the portions y of the curve, located between successive horizontal line signals, are undesired transient signals which are produced during the horizontal return line period. At the point e on the curve. one picture frame has been completed and the cathode-ray is being returned to begin the next scanning of the picture.

It will be apparent that the zero axis for each horizontal line picture signal represented at :c slopes downwardly while the zero axis for the entire group of picture signals of one frame slopes upwardly. It will be apparent that, with the black signal in the direction indicated on the drawings, the signal will result in a shading at the receiver such as shown in Fig. 6, it being understood that the uorescent screen 25! is scanned from top to bottom and (when lookin at the picture) from left to right.

in order to shade the picture evenly, the sawtooth voltage waves shown in Figs. 9 and 1l are mixed with the picture signal, the saw-tooth waves shown in Fig. 9 occurring at the horizontal scanning frequency and the saw-tooth waves shown in Fig. l1 occurring at the frame frequency. The lirst saw-tooth voltage corrects the picture signal to produce a signal such as shown in Fig. 10, while the other saw-tooth voltage further corrects the picture signal to produce the signal shown in Fig. 12. this curve having a zero axis which is horizontal.

It will be understood that where shading of the character shown in Fig. 7 is present, as is usually the case, sine waves also will be added to the picture signal to shade the picture evenly. However, it is not thought desirable to complicate lthe explanation of the invention by drawing curves to illustrate shading by the use of sine waves. Therefore, the type of shading which may be compensated by only two saw-tooth waves has been assumed.

Referring to Fig. 3, it will be seen that the two shading voltages and the picture signal, in-

' dicated at 269, are introduced into the input circuit of the rst amplifier tube ddl where they add to give the curve shown in Fig. 12. The polarity of the signals is reversed by the rst 4sunplier tube as indicated by the curve 2li (Fig. 3).

The polarity of the signal is again reversed by the second amplifier tube its to produce an amplied signal similar to the one shown in Fig. l2. This signal, and blanking impulses oi the character shown in Fig. 13, are impressed upon the input circuit of the third amplier tube 265 where they add to give a signal represented by the curve shown in Fig. 14. It will be noted that the undesired transients now appear on the top oi the planting impulses where they are removed from the region of the picture signals.

The signal appears in reversed phase in the output circuit of the ainplier E65 as indicated. by the curve ZlS (Fig. 3) and is then impressed upon the last ampliier 25'?. pulses and superimposed transients are of considarable magnitude and are in a negative direction, they drive the tube Edi beyond the cut-oli point, indicated by the dotted line 2id, and the transients are clipped ou to produce a signal in the output the amplier which is represented by the curve fll shown in Fig. 3 and in Fig. l5.

synchronizing signals of the character indicated in Fig. i6 are impressed upon the output circuit of the amplier 26'3 where they add to the ampliier output to give the signal represented by the curve in Fig. 17. It will be apparent that the undesired transient signals have been entirely eliminated.

Since the blanlring irnt will also be apparent that unless the zero axis of the picture signals were corrected before the signals were impressed upon the clipping tube 261 either some of the transient voltages would not be eliminated or it would be necessary to so limit the output of the cathoderay transmitter tube that the zero axes of its signal output would have less than a predetermined slope.

In transmitting a picture, the procedure is as follows: .An operator at the transmitting station so adjusts the monitor that the picture appearing on the tubeit is rather dim whereby any irregularities in shading will be readily apparent. He then closes such bus bar switches, 13, |09, etc., as he thinks necessary for correcting any uneven shading that is apparent. The potentiometer contacts t9, E06, etc., and the phase shifting devices are then adjusted. An experienced operator can shade a picture properly without much difficulty. Usually the shading changes with the character of the picture being transmitted so that the operator must watch the monitor tube and change certain shading adjustments from time to time.

It will be understood, of course, that in shading a picture other harmonics of the vertical and horizontal deflecting frequencies than those produced by the circuit shown in Fig. 1 may be utilized, if desired.

By employing my invention a cathode-ray transmitter tube may be operated at its maximum sensitivity or eiliciency and, at the same time, a properly shaded picture obtained at the receiver. Also, the signals impressed upon the receiver will be free from transient signals and the like which would impair the quality of the picture.

It will be understood ifrorn the foregoing description that various modiiications may be made in my invention without departing from the spirit and scope thereof, and I desire, therefore, that only such limitations shall be placed thereon as. are necessitated by the prior art .and set forth in the appended claims.

I claim as my invention:

i. In combination, a cathode-ray picture transmitter tube of the type comprising a mosaic of electron emissive capacity elements which are to be scanned by an electron beam to produce pic'- ture signals and undesired spurious signals'which unevenly shade the picture, means for producing shading signals of a. predetermined wave shape, and means for mixing said shading signals with said picture signals for balancing out said spurious signals, said shading signals having the characteristic that they affect the relative illumination in different parts of the picture. l

2. In combination, a cathode-ray picture transmitter tube of the type comprising a mosaic of electron emssive capacity elements which are to be scanned by an electron beam to produce picture signals and undesired spurious signals which unevenly shade the picture, means for producing signalsroi saw-tooth wave shape and signals o sine Wave shape with said picture signals. and means for mixing all of said signals for balancing out said spurious signals to produce a correctly shaded picture.

3. In a picture transmission system, means for generating picture signals and undesired spurious signals which unevenly shade a picture reproduced by said picture signals, means for generating saw-tooth wave signals and sine wave signals which occur simultaneously with said picture signals, means for so adding all of said signals as to balance out said spurious signals, and

signal.

4. In a picture transmission system, the combination of a cathode-ray transmitter tube which generates picture signals of a character which produce .a picture of uneven shading at the receiver due to spurious signals which unevenly shade the picture, and means located at the transmitter for balancing out at least part of said spurious signals to correct said uneven shading, said means comprising means for generating sawtooth and sine wave voltages which occur simultaneously with said picture signals and for adding them to said picture signals.

5. In a picture transmitter, a cathode-ray transmitter tube of the type including a mosaic of electron-emissive capacity elements, means for sweeping the cathode-ray horizontally across said mosaic at a horizontal deilecting frequency, means for deilecting said cathode-ray simultaneously in a vertical direction at a vertical deflecting frequency whereby said mosaic is scanned periodically by said cathode-ray to produce picture signals and undesired spurious signals which unevenly shade the picture, means for producing a correcting signal which occurs simultaneously with said picture signals, said correcting signal having a fundamental frequency equal to one of said deflecting frequencies, and means for so ad- :ling said correcting signal to said picture signals as to balance out said spurious signals.

6. In a picture transmitter, a cathode-ray transmitter tube of the type including a mosaic :f electron emissive capacity elements, means for sweeping the cathode-ray horizontally across said mosaic at a horizontal deilecting frequency, neans for deilecting said cathode-ray simultane- Jusly in a vertical direction at a vertical deflectng frequency whereby said mosaic is scanned rJeriodically by said cathode-ray to produce pic- ;ure signals and undesired spurious signals which mevenly shade the picture, means for producing i correcting signal which occurs simultaneously with said picture signals, said correcting signal caving a fundamental frequency which is equal Lo a whole number multiple of one of said deecting frequencies, and means for so adding laid correcting signal to said picture signals as :o balance out said spurious signals.

7. In a picture transmitter; a cathode-ray ',ransmitter tube of the type including a mosaic of electron emissive capacity elements.'means for I.weeping the cathode-ray horizontally across laid mosaic at a horizontal deflecting frequency, neans for deilecting said cathode-ray simul- ;aneously in a vertical direction at a vertical leflecting frequency whereby said mosaic is acanned periodically by said cathode ray to proluce picture signals and undesired spurious sigials which unevenly shade the picture, means for )roducing at least two correcting signals which Jccur simultaneously with said picture signals, ind means for adding said correcting signals to zaid picture signals for balancing out said spurius signals, one of said correcting signals having L fundamental frequency equal to said horizontal leilecting frequency, and the other of said cor- 'ecting signals having a fundamental frequency equal to said vertical deilecting frequency.

8. In a picture transmitter, a cathode-ray yransmitter tube of the type including a mosaic af electron emissive capacity elements, means or sweeping the cathode-ray horizontally across :aid mosaic at a horizontal deflecting frequency, neans for defiecting said cathode-ray simultane- 8,100,712 means for transmitting the resulting compositeA ously in a vertical direction at a vertical deflecting frequency whereby said mosaic is scanned periodically by said cathode-ray to produce picture signals and undesired spurious signals which unevenly shade the picture, and means for adding a correcting. signal to said picture signals for balancing out said spurious signals, said correcting signal having a saw-tooth wave form and having a fundamental frequency equal to one of said deilecting frequencies.

9. In a picture transmitter, a cathode-ray transmitter tube of the type including a mosaic of electron emissive capacity elements, means for sweeping the cathode-ray horizontally across said mosaic at a horizontal deflecting frequency, means for deflecting said-cathode-ray simultaneously in a vertical direction at a vertical deecting frequency whereby said mosaic is scanned periodically by said cathode-ray to produce picture signals and undesired spurious signals whichl unevenly shade the picture, and means for adding at least two correcting signals to said picture signals for balancing; out said spurious signals. one of said correcting signals having a fundamental frequency equal to said horizontal deflecting frequency, and the other of said correcting signalsv having a fundamental frequency equal to said vertical deecting frequency, said two correcting signals having substantially a sawtooth wave form.

10. In a picture transmitter, a cathode-ray transmitter tube of the type including a mosaic of electron emissive capacity elements, means for sweeping the cathode-ray horizontallyacross said mosaic at a horizontal deflecting frequency,-

means for deflecting said cathode-ray simultaneously in a vertical direction at a vertical deecting frequency whereby said mosaic is scanned periodically by said cathode-ray to produce picture signals and undesired spurious signals which unevenly shade the picture, and means for adding at least two correcting signals to said picture signals for balancing out said spurious signals, one of said correcting signals having a fundamental frequency equal to said horizontal deflecting frequency, and the other of said correcting signals having a fundamental frequency equal to said vertical deflecting frequency, said two correcting signals having substantially a sine wave form.

11. In a picture transmitter, a cathode-ray transmitter tube of the type including a mosaic of electron emissive capacity elements, means for sweeping the cathode-ray horizontally across said mosaic at a horizontal deilecting frequency, means for deilectingsaid cathode-ray'sixnultaneously in a vertical direction at a vertical deecting frequency whereby said mosaic is scanned periodically by said cathode-ray to produce picture signals and undesired spurious signals which unevenly shade the picture, and means for adding a plurality of correcting signals to said picture signals for balancing out said spurious signals, two of said correcting signals having a sawtooth wave form and two of them having a sine wave form, one saw-tooth signal and one sine wave signal having a fundamental frequency equal to said vertical denecting frequency, and another saw-tooth signal and a sine wave signal having a vfundamental frequency equal to said horizontal deflecting frequency.

12. Apparatus according to claim characterized in that means is provided for shifting the phase of said correcting Signal.

13. In a picture' transmitter. a cathode-ray amavis transmitter tube having an output circuit and including means for producing an electrical image of the picture to be transmitted, said tube having a vertical deecting device and a horizontal defiecting device whereby the cathoderay may be deflected vertically and horizontally simultaneously to produce picture signals and undesired spurious signals which unevenly shade the picture, said spurious signals having one sawtooth component occurring at the vertical deecting frequency and having another saw-tooth component occurring at the horizontal deiecting frequency, means for supplying a saw-tooth electrical wave occurring at the vertical deilecting frequency to said vertical deilecting device, means for supplying a saw-tooth electrical wave occurring at the horizontal deecting frequency to said horizontal defiecting device, and means for adding signals derived from said last two means to the signals appearing in said output circuit for balancing out said spurious signals.

14. In a picture transmitter, a cathode-ray transmitter tube having an output circuit and including means for producing an electrical image of the picture to be transmitted, said tube having a vertical deflecting device and a horizontal deiiecting device whereby the cathoderay may be deflected vertically and horizontally simultaneously to produce picture signals and undesired spurious signals which unevenly shade the picture, said spurious signals having a sawtooth component occurring at the vertical deflecting frequency and having another sawtooth compcnent occurring at the horizontal deflecting frequency and also having sine wave components, means for supplying a saw-tooth electrical wave occurring at the vertical deilecting frequency to said vertical deilecting device, means for supplying a saw-tooth electrical wave occurring at the horizontal deflecting frequency to said horizontal deflecting device, and means for adding saw-tooth and sine wave signals derivedfrom said last two means to the signals appearing in said output circuit for balancing out said spurious signals.

15. In a picture transmitting system of the type in which a cathode-ray tube generates an output signal which includes picture signals and transients and in which the zero axis of said signals varies as the signals are generated due to undesired spurious signals which unevenly shade the picture, the method of eliminating said transients which consists in mixing correcting signals with said output signal, said correcting signals having a. wave shape such as to balance out said spurious signals and correct for the said variation in the zero axis, mixing electrical impulses with said output signal, said impulses occurring at the same time as said transients and being of suicient magnitude to remove the transients from the region of said picture signals. and clipping said transients from the resulting signal.

16. In a picture transmitter. a cathode-ray transmitter tube of the type including a mosaic ci' electron emissive capacity elements, means for sweeping the cathode-ray horizontally across said mosaic at a horizontal deecting frequency, means for deflecting said cathode-ray simultaneously in a vertical direction at a vertical deflecting frequency whereby said mosaic is scanned periodically by said cathode-ray to produce picture signals and undesired spurious signals which unevenly shade the picture, means for producing simultaneously with said picture signals a sine wave signal which has a frequency equal to the second harmonic of one of said deilecting frequencies, and means for adding said sine wave signal to said picture signal for balancingout said spurious signals.

17. Apparatus according to claim 16, characterized in that means is provided 'for shifting the phase of said sine wave.

18. Apparatus according to claim 5, characterized in that means is provided for reversing the polarity of said correcting signal.

AIDA V. BEDFORD. 

